Summary

"We're short-term thinkers: this fiscal quarter, this presidential term. But as Curt Stager points out in this fascinating, eloquent, and truly novel new account, what we're doing in the short run will have an almost unimaginably long-term effect. Our short moment on earth is changing the planet essentially forever, at least in human terms -- which means that this is both a scientifically and ethically crucial book."--Bill McKibben, author ofEaarth "Fascinating and measured - at last someone is taking the long view"--Mark Lynas, author ofSix Degrees: Our Future on a Hotter Planet"A highly entertaining, carefully balanced, and deeply sobering look at our climate future"--William F. Ruddiman, author ofPlows, Plagues, and Petroleum: How Humans Took Control of Climate"While most of our attention in the field of climate science has focused on what we might expect by the end of the 21st century, Curt Stager takes us to a time 100,000 years in the future. This intriguing and thought-provoking view of the far future is an essential read for all interested in the full force of climate change. Without thinking outside the box we would have assumed that abrupt climate change was impossible and that human influence on climate could be transparently absorbed by the massive Earth system. Thinking and experimenting outside the box has expanded our climate view horizons and so will Curt Stager's book."--Paul Andrew Mayewski, Director of the Climate Change Institute, and author ofThe Ice Chronicles

Author Biography

CURT STAGER is an ecologist, paleoclimatologist, and science writer with a Ph.D. in biology and geology from Duke University. He has published more than three dozen climate- and ecology-related articles in major journals including Science and Quaternary Research, and has written for popular audiences in periodicals such as National Geographic. He teaches at Paul Smith’s College in the Adirondack Mountains of upstate New York and holds a research associate post at the University of Maine’s Climate Change Institute, where he investigates the long-term history of climate in Africa, South America, and the polar regions.

For more info, visit www.curtstager.com.

Table of Contents

Acknowledgments

p. xi

Prologue

p. 1

Stopping the Ice

p. 13

Beyond Global Warming

p. 29

The Last Great Thaw

p. 49

Life in a Super-Greenhouse

p. 67

Future Fossils

p. 86

Oceans of Acid

p. 102

The Rising Tide

p. 118

An Ice-Free Arctic

p. 139

The Greening of Greenland

p. 162

What About the Tropics?

p. 181

Bringing It Home

p. 203

Epilogue

p. 228

References

p. 243

Index

p. 271

Table of Contents provided by Ingram. All Rights Reserved.

Excerpts

1

Stopping the Ice

One can only hope that the expected extremes of the Anthropocene will not lead to conditions that cross the threshold to glaciation.—Frank Sirocko, paleoclimatologist.

Shockingly long-term climatic changes await us as a result of modern human activity, but examining our effects on the deep future also raises a related question that is well worth considering: what would global climates have been like if we had left our fossil fuels in the ground rather than burning them?In that alternative reality our descendants would still fret about climate, sea levels, and ice caps but the news would read quite differently from that of today. “There’s a massive, destructive climatic change coming, but scientists say that we can stop it if we take appropriate action now. If we go about business as usual, coastal settlements will be destroyed by sea-level shifts and entire nations will be covered with water. Frozen water. But there’s still hope. If we simply burn enough fossil fuels, we’ll warm the atmosphere enough to delay that icy disaster for thousands of years.”I’m talking about the next ice age. When a paleoecologist like myself thinks about global climate change the exercise is as likely to involve visions of ice-sheet invasions as it is to include greenhouse warming. We still don’t know exactly why continent-sized glaciations come and go as they do, but they clearly have a rhythmic quality to them. Natural cyclic pulses take the long line of temperature history and snap it like a whip, looping it into a series of steep coolings and warmings. When viewed from a long-term perspective, major warmings of the past 2 to 3 million years can seem like brief thermal respites when the world came up for air between long icy dives; that’s why we call them “interglacials” rather than something that sounds more normal or permanent. The cyclic pattern also suggests that more ice ages await us in the future, so strongly in fact that climate scientists routinely refer to our own postglacial warm phase that we live in today as “the present interglacial.” Because of this admittedly unusual perspective, many of the paleoecologists I know balance their concerns about modern climate change with “yes, but it could also be a lot worse.”Although such views are rare outside of narrow academic circles, I believe that they belong in the mainstream. Time perspectives long enough to include ice age prevention are not just the stuff of mind games but potentially important aspects of rational planning for our climatic future. In order to appreciate why this is so, however, it helps to look more deeply than usual into the nature of ice ages.The last one began about 117,000 years ago and ended 11,700 years ago. During that long and terrible reign of cold, roughly a fifth of the world’s land surface resembled the icy interiors of Greenland and Antarctica today, especially in the higher northern latitudes. Most of what is now Canada and northern Europe was smothered under immense sheets of slowly creeping ice up to 2 miles (3 km) thick. The sites of today’s Chicago, Boston, and New York were obliterated, and what we now call Long Island is a plowed-up bow wave of detritus that marks the southern limit of the last major ice advance. Entire landscapes sagged under that tremendous weight, pressing down hundreds or even thousands of feet into the planet’s softer innards, and the gritty underbelly of the ice gouged deep scratches and grooves into solid bedrock that still scar the formerly glaciated regions of the world.When you see glacial deposits and ice-scoured rock formations along a northerly roadside or trail, it’s easy to let your imagination strip away the towns and trees and crush your surroundings under great, grinding slabs of ice. I envision it quite often near my home in the Adirondack Mountains of upstate New York. Recently I was reminded of the frozen past when I stepped off a woodland path near Saint Regis Mountain to take a closer look at one of the largest glacial erratic boulders I’ve ever seen.The massive chunk of gray anorthosite was broader and taller than my house, and the prying fingers of winter frost had plucked garage-sized flakes away from its lichen-crusted flanks. They lay in low heaps around the central body of rock like cast-off clothing. The base of the giant perched just high enough above the ground to leave shadowed crawl spaces below that made me think of of crouching hermits and cave bears. Peering into one of them I scanned its dusky floor for signs of residents but saw only earth-colored gravel. Clean, well-sorted, smoothly rounded gravel, just like the stuff in the shallow streambed nearby. Gravel that was never buried under forest soils or leaf litter and that still looked as fresh as it did when melting ice dropped this gigantic sheltering rock on top of it.That primeval scene drew my imagination back to when these mountains were still emerging from their long, lightless imprisonment. The rustling beech, maples, and birches before me faded away, along with the duff and dirt beneath them, exposing a desolate brown wasteland of wet sand and pebbles that glistened under a cold clear sky. Not a tree in sight, not a shrub or flower, not even many lichens on the virgin boulders yet. Cloudy silt-laden streams and molten blue pools sparkled in the low spots, and remnant hill-sized blocks of decomposing ice hunkered down in the deeper hollows, sloughing off layers of dusty surface debris like old dogs shedding winter fur. Far off on the northern horizon lay an unfamiliar range of white, mile-high hills, the sun-scored southern face of the melting ice sheet. The vision lasted only a few moments, but a strong feeling of connection to long-ago times when Big Ice ruled this landscape stayed with me through the rest of my hike that day.The author examining a glacial boulder near Saint Regis Mountain in the Adirondack Park, upstate New York. Kary JohnsonLet’s continue with this imagination game. What if it happened again?Here and now in the Adirondacks we worry, with good reason, about the effects of acid rain, invasive species, and global warming on our local ecosystems. But those problems won’t exterminate every last Adirondack fish and fowl, and even the most extreme case of Anthropocene heating would still leave the land covered with some sort of greenery, if not all the kinds we’re currently used to.A full glacial advance, on the other hand, is a total wipeout. Every lake is bulldozed or smothered under a thick blanket of cobbles, sand, and gravel. Every sugar maple, every golden-tinted trout lily, every tuft of moss heaves up in bow waves of dirt and stones and is crushed to pulp. Every animal with legs or wings flees southward. The Adirondack peaks vanish under a heavy white tide, the iconic ski jumps at Lake Placid topple and are ground to splinters, and every settlement from Saranac Lake to Old Forge is obliterated.Meanwhile, farther north, most of Canada disappears. That includes Quebec City, Montreal, Ottawa, Toronto, Winnipeg, Calgary, and Vancouver, not to mention every wild area from Hudson Bay to Banff. From a human perspective, there’s no place called Canada for tens of thousands of years except in the same sense that a gigantic frosty slab called Antarctica now squats on the South Pole. And out across the Atlantic, advancing walls of white demolish Dublin, Liverpool, Oslo, Stockholm, Copenhagen, Helsinki, and Saint Petersburg, and every settlement on the rocky coastal rind of Greenland is shoveled into the sea by heavy spatulas of ice.With much of the world’s freshwater imprisoned in frozen form on the continents, sea level falls by as much as 400 vertical feet (120 m). The site of every twenty-first-century port is stranded far inland, the long, slender thumb of Florida doubles in width, and the present location of every shallow-water coral reef in the tropics sprouts weeds and trees. The associated cooling weakens monsoons, locking much of Africa and southern Asia into chronic droughts.This is what a climate historian is likely to have in mind when discussing climatic change. Compare it to what most experts expect modern warming to bring us in the Anthropocene future and you’ll understand why a paleoecologist’s panic button might not be so easily pressed.But wait. Isn’t global warming supposed to trigger the next ice age? Isn’t that what we saw happen in the apocalyptic enviro-thriller movie The Day After Tomorrow, in which the greenhouse effect suddenly shuts down climatically important ocean currents in the North Atlantic and triggers a superglaciation?The movie isn’t totally wrong, in that the warm Gulf Stream really does help to keep northwestern Europe from becoming cooler than it already is. It’s part of a huge global conveyor belt system of interconnected currents that draws solar-heated tropical water into the cold surface of the North Atlantic, where it cools off and then sinks for a deep return journey southward. Some scientists worry that future climatic changes could disrupt that conveyor and trigger a sudden regional cooling; hence the movie scene in which a fierce wind seizes Manhattan with remorseless fangs of frost. But as gripping as that storyline is, serious questions remain about the real role of the conveyor in past and future climate change.The engine driving the conveyor goes by several dry technical names, most recently the meridional overturning circulation, or MOC. It is also sometimes called THC, an abbreviation that is in no way connected to marijuana smoking (and tetrahydrocannabinol) but rather, reflects the upgrading of a simpler concept, that of thermohaline circulation, whose basic premise is that changes in temperature and saltiness drive major circulation currents of the oceans.Warm water on the surfaces of the tropical oceans loses moisture to evaporation, which makes it saltier than average seawater. When the Gulf Stream flows from the hot latitudes between West Africa and the Caribbean into the cooler North Atlantic, it doesn’t easily mix with those northern waters because its tropical heat content makes it less dense (warming makes both water and air expand). But the Gulf Stream gradually releases much of that heat into the cooler air over the North Atlantic, and when it finally does chill down its extra load of salt leaves it denser than usual.That extra density makes some of the Gulf Stream water sink beneath the surface and continue its riverlike meanderings at greater depths. By the time it resurfaces, the deep flow has wormed its way around the southern tip of Africa and entered the Indian and Pacific oceans. Back on the surface again, the current recurves back across those oceans, rounds the tip of South Africa, and returns to the North Atlantic, picking up new loads of equatorial warmth along the way. Additional branches also operate in the Southern Ocean and Arabian Sea, adding extra loops to the tortuous path of the global conveyor.There’s a lot more to the picture than that, however, and when illustrations of this common version of the THC concept appear in professional slide presentations, they can become what one speaker at a recent meeting of the British Royal Society called “oceanographer detectors,” because they make specialists in the audience “go visibly pale at the vast oversimplification.”The THC model is not so much wrong as incomplete. Most scientists have now switched the focus of ocean-climate discussions to the more comprehensive MOC formulation because temperature and salinity aren’t the only drivers of ocean currents after all; winds and tides are at least as influential. THC-style flow does occur, but midlatitude westerly winds and tropical easterly trades do much of the actual pushing.So why does marine MOC affect climate? As heat rises into the air from the Gulf Stream, it warms the westerly winds that blow toward Europe. Without those ocean-tempered winds, London might be as cold as … well, look at a map to see what lies at the same latitude on the opposite side of the Atlantic, and you’ll find snowy Labrador.With this basic introduction to the topic, you’re already well enough equipped to take a pot shot at The Day After Tomorrow. The prevailing winds over Manhattan blow offshore toward the Atlantic, not from it, so why should a Gulf Stream shutdown freeze the city? The film also unrealistically subjects Europe to severe winter conditions year-round. Even if it really did become a climatic equivalent of Labrador, northern Europe would still warm up quite a bit in summer, just as Labrador does.In reality, a MOC slowdown alone couldn’t turn Europe into a climatic twin of Labrador because it lies downwind of a temperature-modulating ocean rather than the interior of a continent. And because prevailing winds spin the North Atlantic surface current system clockwise regardless of what the salinity or temperature of the water is, some version of the Gulf Stream will exist as long as these winds continue to blow over it.Although some computer models do simulate moderate conveyor slowdowns in a warmer future, a truly severe disruption would require extremely large floods of freshwater to pour into the sea, presumably from the melting of land-based ice. If, say, a major ice sheet were to slide off into the North Atlantic where some critical sinking zone is operating, then perhaps it might cap the ocean off with dilute, buoyant meltwater.In 1999, oceanographer Wallace Broecker published a striking theoretical description of just such a total MOC collapse under perfect-storm conditions. Tundra replaces Scandinavian forests. Ireland becomes the climatic equivalent of Spitsbergen, an island in the Norwegian Arctic. When climate modelers working at Britain’s Hadley Center several years ago told their computers to “kill the MOC,” the virtual air outside their lab cooled by 8°F (5°C) within ten years, at least on the digital screen.But Broecker maintains that such a scenario is unlikely today, because those theoretical events only played out in a world that had already been cooled by a prolonged ice age. Nowadays, however, we don’t have nearly enough readily meltable ice left in the Northern Hemisphere to do the job. To reset that stage we’d have to cover Canada, northern and central Europe, and Scandinavia with thick ice caps, and that would require colder, rather than warmer, conditions in the future.Most computer models that have been upgraded so they more accurately represent the role of winds in ocean circulation foresee little, if any, cooling in the North Atlantic region from MOC disruptions during the Anthropocene. As the latest Intergovernmental Panel on Climate Change (IPCC) report concluded, “it is very unlikely that the MOC will undergo a large abrupt transition during the 21st century,” and most experts believe that future greenhouse warming will overwhelm any minor regional effects related to MOC. In light of such findings, Broecker has tried to tamp down some of the worst exaggerations of the ocean-climate link that have been made by nonspecialists, but it’s a tough struggle that pits scientific restraint against the lure of a good story.One case in point is a study commissioned by the U.S. Department of Defense that presented a wildly extremist view of MOC collapse as a grave and imminent threat to national security. In their 2003 report, the authors noted that they were presenting only the most severe of all possibilities, as is commonly done in military planning circles, but that disclaimer was easily missed amid the frightening scenarios that followed. In their depictions, global average temperature shoots up faster and faster until, in 2010 AD, the MOC begins to collapse. Less than ten years later, according to their model, northern Europe cools by 5 to 6°F (3°C), devastating drought strikes the United States, and “a cold and hungry China peers jealously across the Russian and western borders at energy resources.”In response, Broecker wrote an open letter for publication in Science that expressed his dismay over the hyperbole. “I take serious issue with both the timing and the severity of the changes proposed,” he wrote, pointing out that such extreme changes would take a long time to develop and would require glacial-type conditions, not global warming, to trigger them. Furthermore, he cautioned that computer models still can’t fully reconstruct complex MOC disturbances of the past, much less those of the future. He concluded his letter with this admonition: “Exaggerated scenarios serve only to intensify the existing polarization over global warming.”Nonetheless, the idea of a total collapse of MOC is so emotionally gripping that it has become firmly lodged in public consciousness. In that context, oceanographers are watching closely for signs of conveyor responses to modern warming, just to be on the safe side. In 2005, for example, a team of British researchers described a 30 percent slowdown in MOC flow since 1957. The news caught fire among lay and professional audiences alike, but follow-up studies found that the slowdown alert was a “false alarm,” as Richard Kerr put it in a deflating news brief forScience. The pattern of MOC flow is extremely variable, and a more careful look at the numbers showed that the reported trend was indistinguishable from random fluctuations.If MOC changes are unlikely to freeze Europe, will ice ages play any role at all in an Anthropocene world? The answer is a qualified yes, but if they do reappear, it won’t be the fault of ocean circulation disruptions. The main drivers of large-scale glaciation are the movements of Earth itself as it makes its annual elliptical dance around the sun.The way many descriptions read in the popular media, you’d think that stopping our greenhouse gas emissions would prevent climatic change altogether. In fact, climates will always change whether we exist or not, just as they have even on Mars where similarly periodic frosts, thaws, and floods have left their signatures in deposits of red sand, gravel, and dust. Fortunately, because much of that change is cyclical, we can predict some of it by letting it play out on computer screens.The fastest moves in the planetary dance are wobbles. Imagine a whirling top as it slows down and progressively loses its balance. The top begins to dip and swing around in progressively wider circles as it spins more slowly on its axis. Earth does that, too, though not because it’s going to tumble over any time soon. In the wobble cycle, the North Pole draws a full loop roughly every 21,000 years (technically speaking, there are actually two modes of the cycle, 19,000 and 23,000 years long).This has climatic effects because it changes how sunlight intensity, or insolation, affects different parts of the planet’s curved surface. Each year, winter comes to the Northern Hemisphere when it leans away from the sun, and northern summer reigns when it leans toward the sun. Every 21,000 years or so, the wobble cycle brings the Northern Hemisphere its annual dose of summer only when we’re farthest away from the sun on our egg-shaped orbit. When that happens, northern summers become slightly cooler than usual and less snow melts as a result.Keep in mind here that these events don’t happen because the sun itself changes. Instead, it’s because sunlight affects the seasons and hemispheres differently over time. Those effects are amplified by geography because most major landmasses are crowded into the northern half of the globe and dry, solid ground accumulates ice sheets more readily than oceans do. For these reasons, ice ages typically begin in the Northern Hemisphere; the last one was born in the ice-friendly insolation target areas of northeastern Canada and northwestern Eurasia.But that’s not the whole story. Several longer cycles also influence the comings and goings of monster ice sheets.As Earth wobbles, it also tilts more or less steeply in ways that amplify seasonal temperature differences. The slower tilt cycle takes 41,000 years for the North Pole to rock back and forth between 22.1 and 24.5 degrees of arc. When this cycle tilts the planetary axis less steeply, it aims each pole less directly at the sun during its seasonal summertime, so it warms less than usual. For reasons as yet unknown, this was the dominant pacemaker of ice age recurrences until about 1 million years ago, when the wobble cycle and a third, even slower cycle joined forces with the tilt cycle.That third pulse, the eccentricity oscillation, changes the shape of Earth’s orbital route around the sun. The path becomes more or less egglike over the course of about 100,000 years; it also changes in other ways every 412,000 years. Because the sun sits off center within that ring, snuggling a bit closer to one end of the oval than the other, Earth’s distance from the sun varies a great deal through the seasonal circuit, and distortions of that route caused by the eccentricity cycle accentuate those changes. When the orbital ring is most distorted, a good deal less solar heat reaches us at the farthest end of the egg.As all these cycles operate at once, they interact in ways that are easily visualized by comparing them to water waves. I first learned about this from my friend and colleague, ice core researcher Paul Mayewski, who directs the Climate Change Institute at the University of Maine in Orono. He explained how most of the ragged jumps and wiggles in his polar climate records originate.“It’s like waves on a lake,” he began. I imagined orderly ranks of swells rolling along under a brisk breeze. “The rising and falling of the main swells are like the slow eccentricity cycle. Now imagine that a motorboat wake joins the pattern. Those waves are smaller and closer together, so they don’t line up perfectly with the larger, wider ones.”I envisioned an irregular, bumpy surface like the ones that I often encountered during the waterskiing days of my youth. It was easy to keep my balance on a single predictable wave pattern, but when the driver doubled back on our trail or another boat dragged its own chop across our path, it spawned a crazed tangle of leaping, plunging waves. Where two crests briefly collided they bounced skyward, and where two troughs briefly met they bounced lower. Add forward and crosswise motions to such a collision zone and, as I can personally attest, a fallen water-skier will bob around in it like a cork.“That’s where a lot of long-term climate variability comes from,” Mayewski continued. “When different cycles occur simultaneously, they harmonize and strengthen each other sometimes and they weaken or cancel each other at other times. And the more cycles you mix into the climate system, the more erratic it becomes.” Mingling like waves through the ages, Earth’s insolation cycles account for a surprising amount of natural climatic instability, and when they produce an exceptionally long and low temperature combination they can trigger a full-scale ice age.These cyclic patterns were worked out by James Croll, a nineteenth-century Scottish scientist, and later refined early in the twentieth century by a Serbian civil engineer, Milutin Milankovitch. The fit between theory and history isn’t perfect, and many of the past’s shorter temperature perturbations had other causes. Furthermore, we still don’t know exactly why northern glaciations have affected so much of the world at once; the slow gyrations of Earth tend to cool northern summers while warming southern ones, so you might expect ice ages to affect only half of the planet at a time. But the basic hypothesis of insolation-driven ice age pacing is still well supported in the geologic record. Take, for example, the history that was recently revealed by one superlong ice core from Antarctica. It represents 800,000 years of climatic change, and it also captures eight glacial cycles, reasonably in line with the 100,000-year rhythm of the eccentricity cycle.Croll and Milankovitch worked with paper and pencil and devoted countless hours to hand-calculating past insolation values, a mind-numbing task that computers now repeat in seconds. But most interesting in the context of future Anthropocene climate are the models that run those cycles forward in time. By tracking Arctic insolation patterns into the future, we can predict when ice ages should come back to haunt us again—or, rather, when they would occur in the absence of our carbon pollution.The wobble cycle is now trying to make northern summers slightly cooler than usual, which is one of the preconditions for launching a new ice age. And if we think of the last 11,700 years of postglacial conditions as just the latest in a series of similar interglacial warm spells, then common sense suggests that we’re about due for a new cold snap.During the 1960s, a temporary global cooling episode and a few scientists who incorrectly attributed it to the return of icehouse conditions launched a brief flurry of “fear the global cooling” excitement in the media. That response, of course, was misguided because the decadal time scale under consideration was far too short to represent Milankovitch-scale cycles, and what at the time may have felt like the start of a neoglaciation was in fact merely a brief pause in the twentieth-century warming trend.But we can do much better than that today. Well-trained specialists now devote their careers to calculating the patterns and climatic effects of orbital cycles. Among the most influential of these are André Berger and Marie-France Loutre, a pair of climatologists at the Institute of Astronomy and Geophysics in Louvain-de-Neuve, Belgium. Their charts and tables of insolation data are widely cited in the scientific literature, and the graph shown here was drawn with data that were kindly provided by Dr. Loutre.Solar insolation at 60 degrees North, showing an anticipated cooling event ca. 50,000 AD that might have triggered the next ice age in the absence of lingering fossil fuel emissions. Data courtesy of Marie-France Loutre.So, when is the next ice age scheduled to arrive? A Berger-Loutre chart that appeared in Science in 2002 lays it out clearly. Today, the Arctic has entered a phase of relatively weak summer insolation that, in earlier times when the eccentricity cycle was stronger, might have been deep enough to call another ice age down upon us within the next few thousand years. But this episode will be too mild to do the job, even without our greenhouse gases working against it. In other words, we’ve just dodged an icy bullet, thanks to a slight, temporary diversion of our route around the sun.Berger and Loutre’s work also shows that the effects of the 412,000-year eccentricity cycle have been weakening in recent millennia, which helps to flatten out the depths of cooling pulses. The more symmetrical our orbit becomes as a result of changes in that longest of cycles, the narrower the range of seasonal temperatures on Earth. This will set an even calmer tone for cyclic climate changes in the more distant future of the Anthropocene.Roughly 25,000 years from now, our orbit will be about as close to circular as it ever gets, and the power of the other cycles to warm or cool the ice masses of the Arctic will be as weak as it ever gets. Under those conditions, we’ll still face brief regional disturbances, such as the North Atlantic Oscillation, El Niño, and the sheer orneriness of local weather that we know today, but they’ll only raise short-lived climatic ripples on the flattened surface of the longer orbitally driven patterns. With the gentle urging of slightly higher axial tilt than we experience today, the curve of northern summer insolation will simply add a very mild rise to a puny peak—more like a low hilltop—about 25,000 years from now.Although that low blip will cause only a mild thermal boost, it nevertheless does mean that Arctic summers were already slated to warm very slightly in coming millennia even without our influence. If Berger and Loutre are correct, then this may be unwelcome news to fans of low sea levels and heavy ice cover on Greenland.According to the calendar of orbital cycles, the next serious risk of an ice-spawning chill during Arctic summer isn’t due until about 50,000 AD, but here is where we humans enter the scene. Most climate models will only trigger an ice age at that distant point in future time if the CO2 content of the atmosphere is no higher than 250 parts per million (ppm). With CO2 concentrations as of this writing at 387 ppm and rising, they clearly won’t be going back below that critical threshold for a very long time. In fact, as will be explained in Chapter 3, those values will still hang well above 250 ppm in 50,000 AD, and they won’t return to such preindustrial conditions for tens of thousands of years beyond that.These two factors together—the inherent weakness of the next cooling cycle and the longevity of our carbon pollution—lead to one astounding conclusion. Not only have we warmed the world during this century with our carbon emissions; we’ve also stopped the next ice age in its tracks.According to recent computer model studies, this may be only the beginning of our influence over the coming and going of ice ages, depending on what we do during the next century or two. If we hold ourselves to a relatively moderate sum total of fossil fuel emissions before switching to alternative energy sources, then we skip the next glaciation in 50,000 AD. But more of those natural cooling pulses will develop later on. When the next big one strikes the Northern Hemisphere some time around 130,000 AD, the greenhouse pollution legacy of a moderate-emissions scenario will have faded to insignificance, leaving nothing to resist glaciation. Once again, ice will bury the northern sectors of the northern continents. Or rather, it will do so unless we burn through all of our remaining coal reserves during the next century or so. If we go down that profligate path instead, there won’t be any ice ages for a very long time: not in 50,000 AD, not in 130,000 AD, and not during the other insolation minima of the next half million years.How should we react to this news? When I imagine our carbon footprint kicking the door closed on an ice age, my initial reaction is one of shock mingled with a twinge of fear. But a second, rather confusing response follows when I imagine our carbon emissions saving the northern United States, Canada, and much of northern Eurasia from being crushed under gigantic ice sheets.It would be stretching the bounds of credibility to say that greenhouse pollution is anything but a vexing problem. But on the scale of planetary disaster, an ice age is to global warming as thermonuclear war is to a bar brawl. We’re certainly justified in worrying about the environmental disturbances that are coming our way, but even this situation is arguably better than losing entire nations and ecosystems to total icy destruction. What if we had to choose between the two options?In fact, we do face that very choice as we weigh our possible responses to modern climate change. Most of the more extreme negative effects of today’s fossil fuel emissions will be felt within the next millennium or so, but what if those same fumes later go on to save much later generations from having to endure otherwise inevitable ice ages?At first, this ethical dilemma might seem outlandish, a silly kind of joke. Speaking of it too flippantly feels disrespectful of those who may suffer from the climatic changes of today and in the near future. It also feels like tossing raw meat to the habitual contrarians who seek any excuse to avoid controlling fossil fuel consumption. But the facts are plain, and I believe they’re worth considering carefully.Unfortunately, the large time scales involved in this subject complicate discussions of it. It’s much easier to imagine what climate change might do to ourselves or our grandkids than what it might do to people 130,000 years from now, and this can make it difficult for some individuals to take the matter seriously. For them, weighing the value of their own lifetimes against those of unknown citizens of the far future is as ridiculous as weighing the value of a living child against that of a cartoon character; such distant people and times are simply too remote to seem real.On the other hand, many of us do enter that deep pool of inquiry when we consider doing things for the sake of future generations, though our entrance is usually more of a toe dabble than a dive. In fact, thinking and acting on behalf of future-dwellers isn’t the simple endeavor that it may seem to be, especially if you don’t restrict your attention to those closest to us in space and time.For one thing, you have to decide who those beneficiaries will be. Does your list include everybody who will be alive in, say, 2500 AD or just a select subset of them? You might be willing to give up your gas-guzzling car in order to keep the climate of a direct descendant’s world similar to that of your own. But what if that choice also harms someone else, perhaps by making some future nation much wetter or drier than it would otherwise be: will you favor your own lineage over those of others? And if you choose to favor your own descendants, you still have to decide which ones to focus on because various generations will prefer different things at different places and times in the future.If we imagine the Anthropocene in its entirety, it gives us a very large and diverse collection of future people to consider. Many of them will inhabit a warming world, but most will live on the long cooling tail-off of atmospheric CO2 recovery. Perhaps citizens of 130,000 AD might decide that, after so many millennia of ice-free oceans and high-latitude commerce, they’d rather keep things the way they are rather than let CO2 concentrations return to preindustrial levels, particularly with a new ice age looming. Maybe by then our long-lived carbon emissions will have begun to seem less like pollution and more like insurance against global cooling.Then again, maybe not. But who are we to decide their fate, anyway?We will be messing with planetary temperatures for a very long time, well past the temporal limit of 2100 AD, but we still have time to head off the most extreme consequences if we choose to do so with a long-term view of the future in mind. One of the most important decisions that our generation will ever make is the critical choice between a relatively moderate scenario in which we quickly replace fossil fuels with other energy sources and an extreme one in which we burn through our remaining cheap petroleum and then go on to consume our coal reserves as well. But in either case, one thing is becoming increasingly clear. Humankind is going to face a long-lasting array of environmental challenges of its own making as the Anthropocene runs its course.